Natural sounds, including those used for communication by humans, are complex signals that often contain many frequencies that flucturate in amplitude and time. These fluctuations are known to be information-bearing elements of such sounds and the auditory systems of some animals have apparently evolved mechanisms for their detections and transmission. Auditory cortex of the cat comprises a number of fields that are distinct from one another in structure and function. The primary auditory area (AI) has been studied extensively as regards the mechanisms of processing simple acoustic stimuli. Little is known about the ways that it processes more complex signals such as frequency- and amplitude-modulated signals which are major components of natural sounds. Essentially nothing is known about the development of these mechanisms in early postnatal life when the auditory system is maturing structurally or the effects on them of altered acoustic environments or cochlear lesions. The long term objective of this research is to understand the ways in which auditory areas of cerebral cortex normally process such complex sounds, how this processing develops after birth in a maturing auditory system, and how this processing might be altered by altering the input to the auditory system either by sound conduction blockade or receptor damage. The responses of single auditory cortical neurons are recorded with extracellular microelectrodes in anesthetized animals when controlled complex sounds are delivered to one or both ears. Amplitude- or freqency- modulated tones or noises are the principal stimuli used to mimic some of the salient features of natural sounds. The spatial distribution of neurons sensitive to particular attributes of such sounds is mapped on to the cortex and related to known functional organizations and patterns of anatomical connections. Similar experiments are carried out on animals reared from birth with conductive hearing loss or unilateral cochlear destruction. This work should reveal cortical mechanisms involved in complex sound analysis and thereby help in understanding speech perception problems associated with central nervous system disorders.